I. Field
The disclosed embodiments pertain generally to the field of wireless data communications, and more specifically to a method and apparatus for controlling vocoder frame generation in a discontinuous transmission communication system.
II. Background
Wireless communications have become commonplace in much of the world today. In many digital wireless communication systems, audio information, typically voice, is transmitted between wireless communication devices and other end units via infrastructure equipment. Examples of various communication systems include code division multiple access (CDMA) systems, global system for mobile communications (GSM) systems, wideband code division multiple access (WCDMA) systems, as well as others.
In many wireless communication systems, human speech is converted into electronic signals and digitized. The digitized speech is often provided to a vocoder, which is a well known device in the art for compressing the digitized speech signal for efficient wireless transmission. The output of the vocoder comprises vocoder frames, which are discrete “packages” of bits representing the compressed digitized speech. Vocoders may operate using either fixed or variable rate encoding techniques, both of which are well known in the art. In either case, vocoders operate to take advantage of natural pauses, or lapses, inherent in human speech to provide bandwidth compression. In some communication systems using fixed rate vocoders, vocoder frames are not transmitted during periods of speech inactivity, thereby reducing the bandwidth necessary for the communication.
Several problems are inherent in the fixed rate vocoder application. First, the transition from periods of speech activity to periods of speech inactivity may be noticeable to users. Another problem is that the background noise inherent in most telephonic communications is not preserved as the communication transitions from periods of speech activity to periods of speech inactivity. These problems are exacerbated in communication systems employing secure communication techniques, such as public key encryption techniques.
In a fixed rate vocoder application, it would be desirable to preserve the background noise during such transitions so that users do not perceive noticeable sound quality differences.